/*-------------------------------------------------------------------------
 *
 * sortsupport.c
 *      Support routines for accelerated sorting.
 *
 *
 * Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *      src/backend/utils/sort/sortsupport.c
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "access/nbtree.h"
#include "catalog/pg_am.h"
#include "fmgr.h"
#include "utils/lsyscache.h"
#include "utils/rel.h"
#include "utils/sortsupport.h"


/* Info needed to use an old-style comparison function as a sort comparator */
typedef struct
{
    FunctionCallInfoData fcinfo;    /* reusable callinfo structure */
    FmgrInfo    flinfo;            /* lookup data for comparison function */
} SortShimExtra;


/*
 * Shim function for calling an old-style comparator
 *
 * This is essentially an inlined version of FunctionCall2Coll(), except
 * we assume that the FunctionCallInfoData was already mostly set up by
 * PrepareSortSupportComparisonShim.
 */
static int
comparison_shim(Datum x, Datum y, SortSupport ssup)
{
    SortShimExtra *extra = (SortShimExtra *) ssup->ssup_extra;
    Datum        result;

    extra->fcinfo.arg[0] = x;
    extra->fcinfo.arg[1] = y;

    /* just for paranoia's sake, we reset isnull each time */
    extra->fcinfo.isnull = false;

    result = FunctionCallInvoke(&extra->fcinfo);

    /* Check for null result, since caller is clearly not expecting one */
    if (extra->fcinfo.isnull)
        elog(ERROR, "function %u returned NULL", extra->flinfo.fn_oid);

    return result;
}

/*
 * Set up a shim function to allow use of an old-style btree comparison
 * function as if it were a sort support comparator.
 */
void
PrepareSortSupportComparisonShim(Oid cmpFunc, SortSupport ssup)
{
    SortShimExtra *extra;

    extra = (SortShimExtra *) MemoryContextAlloc(ssup->ssup_cxt,
                                                 sizeof(SortShimExtra));

    /* Lookup the comparison function */
    fmgr_info_cxt(cmpFunc, &extra->flinfo, ssup->ssup_cxt);

    /* We can initialize the callinfo just once and re-use it */
    InitFunctionCallInfoData(extra->fcinfo, &extra->flinfo, 2,
                             ssup->ssup_collation, NULL, NULL);
    extra->fcinfo.argnull[0] = false;
    extra->fcinfo.argnull[1] = false;

    ssup->ssup_extra = extra;
    ssup->comparator = comparison_shim;
}

/*
 * Look up and call sortsupport function to setup SortSupport comparator;
 * or if no such function exists or it declines to set up the appropriate
 * state, prepare a suitable shim.
 */
static void
FinishSortSupportFunction(Oid opfamily, Oid opcintype, SortSupport ssup)
{
    Oid            sortSupportFunction;

    /* Look for a sort support function */
    sortSupportFunction = get_opfamily_proc(opfamily, opcintype, opcintype,
                                            BTSORTSUPPORT_PROC);
    if (OidIsValid(sortSupportFunction))
    {
        /*
         * The sort support function can provide a comparator, but it can also
         * choose not to so (e.g. based on the selected collation).
         */
        OidFunctionCall1(sortSupportFunction, PointerGetDatum(ssup));
    }

    if (ssup->comparator == NULL)
    {
        Oid            sortFunction;

        sortFunction = get_opfamily_proc(opfamily, opcintype, opcintype,
                                         BTORDER_PROC);

        if (!OidIsValid(sortFunction))
            elog(ERROR, "missing support function %d(%u,%u) in opfamily %u",
                 BTORDER_PROC, opcintype, opcintype, opfamily);

        /* We'll use a shim to call the old-style btree comparator */
        PrepareSortSupportComparisonShim(sortFunction, ssup);
    }
}

/*
 * Fill in SortSupport given an ordering operator (btree "<" or ">" operator).
 *
 * Caller must previously have zeroed the SortSupportData structure and then
 * filled in ssup_cxt, ssup_collation, and ssup_nulls_first.  This will fill
 * in ssup_reverse as well as the comparator function pointer.
 */
void
PrepareSortSupportFromOrderingOp(Oid orderingOp, SortSupport ssup)
{
    Oid            opfamily;
    Oid            opcintype;
    int16        strategy;

    Assert(ssup->comparator == NULL);

    /* Find the operator in pg_amop */
    if (!get_ordering_op_properties(orderingOp, &opfamily, &opcintype,
                                    &strategy))
        elog(ERROR, "operator %u is not a valid ordering operator",
             orderingOp);
    ssup->ssup_reverse = (strategy == BTGreaterStrategyNumber);

    FinishSortSupportFunction(opfamily, opcintype, ssup);
}

/*
 * Fill in SortSupport given an index relation, attribute, and strategy.
 *
 * Caller must previously have zeroed the SortSupportData structure and then
 * filled in ssup_cxt, ssup_attno, ssup_collation, and ssup_nulls_first.  This
 * will fill in ssup_reverse (based on the supplied strategy), as well as the
 * comparator function pointer.
 */
void
PrepareSortSupportFromIndexRel(Relation indexRel, int16 strategy,
                               SortSupport ssup)
{
    Oid            opfamily = indexRel->rd_opfamily[ssup->ssup_attno - 1];
    Oid            opcintype = indexRel->rd_opcintype[ssup->ssup_attno - 1];

    Assert(ssup->comparator == NULL);

    if (indexRel->rd_rel->relam != BTREE_AM_OID)
        elog(ERROR, "unexpected non-btree AM: %u", indexRel->rd_rel->relam);
    if (strategy != BTGreaterStrategyNumber &&
        strategy != BTLessStrategyNumber)
        elog(ERROR, "unexpected sort support strategy: %d", strategy);
    ssup->ssup_reverse = (strategy == BTGreaterStrategyNumber);

    FinishSortSupportFunction(opfamily, opcintype, ssup);
}
